Effects of magnetic ordering on dynamical conductivity: Optical investigations of EuFe2As2 single crystals

The magnetic, transport, and optical properties of EuFe2As2 single crystals have been investigated parallel and perpendicular to the ab plane. The anisotropy rho(c)/rho(ab)approximate to 8 depends only slightly on temperature. In both orientations, the spin-density-wave transition at T-SDW=189 K shows up as a considerable increase in the dc resistivity. Susceptibility measurements evidence the magnetic order of the Eu2+ moments at T-N=19 K with little influence on the electronic transport taking place in the FeAs layers. Polarization-dependent infrared spectroscopy reveals strongly anisotropic optical properties and yields a carrier density of only 4.2x10(21) cm(-3) and a bandmass of m(b)=2m(0). A sizeable Drude contribution is present at all temperatures and narrows upon cooling. Below T-SDW, the spin-density-wave gap develops in the in-plane optical conductivity; no appreciable change is detected for the perpendicular polarization. Modifications in the phonon features are associated with changes in the electronic properties at T-SDW. The extended Drude analysis yields a linear behavior of the frequency-dependent scattering rate below T-SDW, indicating an interaction between the charge carriers and spin fluctuations in the spin-density-wave state.